Why Does My Candle Have Sinkholes or Cracks After It Sets?

Wax shrinks as it cools, and air trapped in the wax has to escape before the top sets. When the surface sets first, the wax underneath pulls away and leaves a hole or a crack. CandleScience says the cause is air trapped in the wax that rises but cannot escape before the surface sets, leaving a void that collapses the surface above it. Both can be fixed on a set candle, and both point back to how the candle cooled.

What is the difference between a sinkhole and a crack?

A sinkhole is a dip or hole in the top. CandleScience says they can be very small or as deep as an inch (2.5 cm). A crack is a split in the surface. CandleScience calls a thin crack in a circle about 1 inch (2.5 cm) from the wick common, caused by slight shrinkage and tiny air bubbles at the bottom of the container, and says it does not affect the burn. Lone Star Candle Supply describes a small crater that forms near the wick as the wax cools, because the shrinkage concentrates there. If the top is rough or bumpy instead, see our guide on rough tops. Frosting and wet spots are a different fault, covered in the frosting guide.

How do I fix a candle that has already set?

1. Let it set fully. Lone Star says to wait until the candle has fully set before the second pour.
2. Poke the sinkhole. CandleScience says to poke it a few times with a skewer to release trapped air. Makesy also suggests relief holes.
3. Choose your fix. Either do a second pour, or use a heat gun.
4. Second pour: re-melt wax you held back, to your pouring temperature, and pour slowly over the top. CandleScience says about 1/8 inch (3 mm), with enough of a layer to fill the holes.
5. Heat gun instead: CandleScience says it may take 1 to 2 passes to smooth the surface. Makesy's smooth-tops article says to use an entry-level heat gun, keep it away from the wick, and not to linger in one spot, since the fragrance can burn off and weaken the cold throw.
6. Leave it alone while it sets again. Makesy says not to move the candle after heating, so the top stays flat.

Makesy says never to use a blow dryer, because it blows melted wax around. CandleScience's smooth-tops page says a hair dryer can work in a pinch but its wide airflow can throw hot wax. I would use the heat gun.

How do I stop it happening next time?

• Cool slowly and steadily. Lone Star says rushing with a fridge, freezer or fan is how wet spots and cracks happen, and its soy FAQ says speeding up the cool may crack the top more. CandleScience says to cool on a wire rack.
• Do not fill past the widest part of the jar. Lone Star's FAQ says the top then cools faster than the inside and may sink. See our guide on how much wax to fill a jar.
• Adjust the pour temperature by 5 degrees. CandleScience's advice for cracks is to try 5 degrees either way. Its sinkhole section does not mention pour temperature. Lone Star says a cracking candle may have been poured too hot or too cool. Start from your wax maker's sheet. See our pour temperature guide.
• Tap the jar gently after pouring to release bubbles, as CandleScience says.
• Warm the jar. Lone Star says warm jars reduce cracking when hot wax meets cold glass and gives a 150 to 170 degrees F (66 to 77 C) oven. CandleScience's soy guide, under frosting, says not to heat containers over 100 degrees F (38 C). The sources disagree, so follow your wax and jar supplier.
• Keep it still while it sets, with no fans or vents, as Lone Star says.

Worked example: a pour temperature trial

Say your wax sheet gives a pour temperature we will call T. Pour 12 jars the same way, with the same wax, jar, wick and cooling spot. Change only the pour temperature: 4 jars at T minus 5 degrees F, 4 at T, 4 at T plus 5 degrees F. The counts below are examples.

Here, cooler was better. The next round starts at T minus 5 and tries T minus 10. Four jars per group is a small sample, so repeat the best setting on a second batch before you trust it.

What does a fix cost? Say 3 jars need a second pour of 0.6 oz of wax each, at an example wax cost of $0.18 an ounce: 3 x 0.6 = 1.8 oz, and 1.8 x 0.18 = $0.32. The wax is cheap. If each fix takes 4 minutes, that is 12 minutes, so your time is the real cost. That is a good reason to fix the cause.

Common mistakes

• Cooling a batch in the fridge or freezer to save time.
• Changing pour temperature, jar and cooling spot all at once.
• Holding no wax back, then having nothing for the second pour.
• Aiming a heat gun at the wick or leaving it in one spot.
• Filling past the widest part of the jar.

Questions

Do sinkholes and cracks change how the candle burns?

CandleScience says a thin crack about 1 inch from the wick does not affect the burn. I did not find a source that says a sinkhole changes it. Test your finished candle in your own jar with your own wick and wax.

Is this a soy wax problem?

CandleScience's soy guide says air can get trapped in soy wax when it is made. Lone Star calls a small crater near the wick normal even with single-pour waxes, and Makesy's sinkhole article suggests blended waxes. I did not find a source that compares waxes, so check your wax maker's own guidance.

Should I sell candles with a repaired top?

That is your call, and I found no rule about it in the sources I read. Whatever you decide, burn-test a repaired candle like any other before you sell that recipe.